Self-regulating heater systems operate a heating element or elements at a controlled temperature without the necessity of a separate temperature sensor, such as a thermocouple or thermostat. Such a system utilizes a heater element which exhibits a large temperature coefficient of resistance (TCR), i.e., a large change in the resistivity of the heater element for a relatively small change in the temperature of the heater element, in the temperature range at which the heater system is intended to operate. The heater element may have a negative temperature coefficient of resistance (NTCR) in which the resistivity of the heater element decreases as temperature increases or a positive temperature coefficient of resistance (PTCR) in which the resistivity of the heater element increases as temperature increases. Conventionally, during heat up, a heater system employing a NTCR heater element would typically power its heater element at a constant current, while a heater system employing a PTCR heater element would typically power its heater element at a constant voltage.
These self-regulating heater systems indirectly sense the temperature of the heater element by measuring the resistance of the heater element. Therefore, since the resistance of the heater element is known for a particular temperature, the temperature of the hearer element can be determined and regulated. Some prior an self-regulating heater systems operate at only one specific temperature as determined by the TCR properties of the heater element. Consequently, when it is desired to operate the system at a different temperature, the heater element must be replaced. Other prior art systems continuously monitor the resistance of the heater element and adjust the voltage or current to the heater element to provide a small degree of temperature settability. Some of these systems supply either a constant voltage or constant current until the heater element reaches the set temperature, whereupon the power supply converts to resistance control to maintain the temperature at this set point. A need exists, however, for heater systems that have shorter heat up times and better temperature control than that afforded by prior art systems.